Olfactomedin 4 down-regulates neutrophil killing of Gram-positive and Gram-negative bacteria We recently demonstrated that OLFM4 negatively regulates host innate immunity against Helicobacter pylori. To extend our previous study, we determined whether OLFM4 might regulate innate immunity against broad bacteria infections by investigating the bactericidal activity in neutrophils from OLFM4-/- mice. In this study, we have demonstrated that OLFM4 is a novel neutrophil granule protein. Neutrophils from OLFM4-/- mice have increased intracellular killing of Staphlocococus aureus and Escherichia coli in vitro. The OLFM4-/- mice have enhanced in vivo bacteria clearance and are more resistant to sepsis when challenged with intra-peritoneal injection of S. aureus or E. coli. Using a yeast two hybridization assay, OLFM4 is found to interact with cathepsin C, a cysteine protease that pays an important role in the bacterial clearance and inflammatory response. We have demonstrated that OLFM4 is a direct substrate of cathepsin C and inhibits cathepsin C activity in vitro and in vivo. The cathepsin C activity in neutrophils from OLFM4-/- mice is significantly higher than that in neutrophils from wild-type littermate mice. The activities of serine proteinases, neutrophil elastase, cathepsin G and proteinase 3, whose processing and maturity require cathepsin C activity, are also higher in OLFM4-/- neutrophils. These results indicate that OLFM4 is an important negative regulator of neutrophil bactericidal activity against broad range of bacteria via restricting cathepsin C activity and its down stream granule-associated serine proteases. Olfactomedin 4 is a secreted antagonist of Wnt/&#946;-catenin pathway OLFM4 is a secreted glycoprotein, but its effect on intracellular signal pathway has not been investigated. Previous studies have shown that members of olfactomedin family, olfactomedin 1 and myocillin are modulators of Wnt signaling pathways. Here we investigated whether OLFM4 can regulate Wnt pathway. To address this question, we treated 293T cells with purified OLFM4 protein. Downregulation of &#946;-catenin protein levels were observed in total cell lysate and more obviously in subcellular fractions including cell membrane, cytoplasm and nucleus. The degradation of &#946;-catenin was dependent on GSK3-mediated phosphorylation and go through proteasome mediated degradation. Overexpression of OLFM4 in 293T cells also decreased the &#946;-catenin level. We have found exogenously added OLFM4 protein and overexpression of OLFM4 can antagonize Wnt3a induced upregulation of &#946;-catenin in the cytoplasm and nucleus of 293T cells. Primary study demonstrated that OLFM4 might interact with Frizzled receptor in 293T cells. We have identified OLFM4 as an extracellular Wnt antagonist that inhibits Wnt signaling probably at the level of interaction with its surface receptors. OLFM4 may prove to be an effective therapeutic agent to interfere with Wnt pathway involving Wnts and their receptors.